Types of molds
A two-plate mold (Figure 24) has only one parting line. If a runner is used, it is connected to the molded product and requires manual removal and separation after the part is ejected. This type of mold is the least expensive to make and is commonly used for parts with relatively simple geometries.
Figure 23. Hydromechanical clamping system. Top view shows clamp open position of piston and ram. Bottom view shows clamp closed, toggled and ram pressurized.
Figure 24. Two-plate mold
Figure 25. Three-plate mold
Three-plate molds (Figure 25) have two parting lines, one for the runner system and one for the molded product. When the mold opens, the runner is automatically separated from the product to allow separate handling. This eliminates the need for manual separation and removal and the sprue and runner system may be fed directly to a recycling system. This type of mold is more expensive than the two-plate mold.
Stack molds (Figures 26 and 27) can be two, three or four levels. The advantage of the stack mold is that it can, generally, produce a larger number of products versus a two or three-plate mold utilizing the same machine clamp tonnage. The disadvantage is that it requires a molding press with much greater daylight opening to accommodate the mold height. This type of mold is much more expensive and takes longer to build. Three level stack molds are very new and four level stack molds have been around for less than five years. The dairy container and lid industries commonly use stack molds. The four-level is common for lids, and the two-level is common for containers.
Figure 26. 2x1 wash basin stack mold
Figure 27. 4x24 stack mold Photo courtesy of Tradesco Mold, Ltd.
Sprues and runners
The sprue and runner system is the conduit that connects the machine nozzle to the cavities. During the injection phase of the molding cycle, the molten material flows through the sprue and runner to the cavities.
The sprue connects the machine nozzle to the runner and may be either a cold or a hot- sprue. In the hot-sprue design, the sprue has heating elements that maintain the plastic in a molten state eliminating the need for separation and reclamation. Ideally, the sprue should be as short as possible to minimize the pressure loss during injection. A cold sprue is tapered for easy release from the mold.
There are three basic runner types in use:
• Cold Runner
• Insulated Runner
• Hot Runner
Cold runners are commonly used in two and three-plate molds, but not in stack molds which require the use of a hot runner. The most commonly used runner designs are fullround, half-round and trapezoidal (Figure 28). The full-round is generally preferred for ease of machining and lower pressure loss. For fast cycles a full-round is not recommended since the greater mass of material takes longer to cool and may control the cycle time. The runner should be polished for ease of mold filling and part ejection.
The insulated runner (Figure 29) is the precursor to the hot runner. The runner diameter is very large and a thick skin is formed on the outside of the runner. The molten plastic flows in the center and, due to external insulation and the low thermal conductivity of the polymer, remains molten during the cycle. This design eliminates the need for removing and/or recycling the runner. The problem with this design is that when the machine is down for any extended period of time the runner solidifies and has to be physically removed before beginning the next molding cycle. As molders have become more comfortable with hot runner technology, insulated runners are rapidly becoming obsolete and not many molds are built today utilizing this technology.
The externally heated hot-runner system (Figure 30) also maintains the plastic in a fluid state while the mold is running with the pressure at each gate approximately the same. Maintaining a uniform temperature in the sprue bar and the hot-runner manifold is very critical to process and product consistency. Start-up procedures must be carefully followed according to the mold maker’s specifications to prevent damage and material leakage in the manifold. 注射成型工艺英文文献和中文翻译(5):http://www.youerw.com/fanyi/lunwen_6568.html